Silicon carbide (SiC) is a semiconductor with certain advantages over pure silicon that make it desirable in high-power, high-frequency and high-temperature applications. Its downside, however, is that good SiC crystals are hard to grow. Researchers in Japan report today in the journal Nature that they have overcome these difficulties by improving the seed on which the crystals form.
Unlike most industrial semiconductors, SiC does not have a liquid form, which means that entirely novel techniques had to be developed to grow large crystals from SiC vapor. Unfortunately these processes result in structural defects that affect the performance of chips made from the resulting SiC wafers. Kazumasa Takatori of Toyota Central R&D Laboratories and his colleagues were able to reduce the number of defects in a single SiC crystal by growing it in multiple directions.
The basic structure of SiC is a silicon-carbon bilayer in which silicon atoms sit on top of hexagons of carbon. These bilayers define what is referred to as the c-face. Researchers generally grow wafers on top of the c-face of a seed crystal, but the end-products tend to be riddled with micrometer-sized holes. To avoid these "micropipes," Takatori and his collaborators grew their crystals on top of a different side--on one of the so-called a-faces. The process is analogous to building a house, not from the ground up, but from the eastern wall to the western wall. The crystals the team initially produced in this manner were actually full of defects, but the scientists cut slices along the "north-south" direction (also an a-face) and used them to grow new crystals. This a-face reseeding was repeated several times, with a final step of c-face growth. The results show that the number of defects in the crystals fell exponentially with each stage of a-face growth.
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Takatori expects that once reliable components are made, SiC could appear in power converters, cars, and household appliances, as well as amplifiers for communication base stations. "SiC semiconductors are generally expected to be put into practical use at around 2010-2012," he says. In an accompanying commentary, Roland Madar from the National Polytechnic Institute in Grenoble, France, notes that, "Silicon carbide has become, at last, a contender for silicon's crown.¿
